docs: def _get_normals(v):
      '''get two vectors that form a basis w/ v
  
      Note: returned vectors are unit
      '''
      not_v = np.array([1, 0, 0])
      if np.all(np.abs(v) == not_v):
          not_v = np.array([0, 1, 0])
      n1 = np.cross(v, not_v)
      n1 /= norm(n1)
      n2 = np.cross(v, n1)
      return n1, n2